It is known to include a plurality of integrated circuits in a single package body as a way to increase the density of integrated circuits on a printed circuit board. U.S. Pat. No. 5,815,372 provides examples of packages having two integrated circuits.
Another method to increase package density is to mount a first small outline integrated circuit package (xe2x80x9cSOICxe2x80x9d) having either gull wing or J lead styles on a printed circuit board. Next, the leads of a second SOIC package are cut so as to form lead stubs. The second SOIC package is then stacked on the first SOIC package, and the lead stubs are soldered to the leads of the first SOIC package. Unfortunately, this method is difficult and time consuming. Moreover, the footprint of the first SOIC package is relatively large due to the gull wing or J-style leads.
U.S. Pat. No. 5,835,988 discloses packages that can be stacked in multiple layers. In view of the teachings of the present invention, however, this design has shortcomings. For example, it appears that the packages of FIG. 12 would be difficult to stack, because no provision is made to align the packages vertically. The embodiment of FIG. 13 employs an external leadframe between the packages, but the number of packages that can be stacked is preordained by the size of the leadframe. Further, the packages are unsuitable for various types of integrated circuits, such as optical integrated circuits, micromachines, and gallium arsenide integrated circuits, because of the encapsulant used for the package body. Moreover, the design does not appear robust, because no provision is made to secure the leads to the encapsulant.
The present invention provides packages that may easily be stacked one on top of another. Stacking the packages allows an increase in the density of packages on a printed circuit board without a corresponding increase in the area of the printed circuit board consumed thereby. Moreover, there are many ways of stacking and/or inter-connecting the packages, which gives the user great flexibility.
One embodiment of a stackable package within the present invention includes a package body having a first surface, side surfaces extending vertically from said first surface, and a hollow central cavity having an opening opposite said first surface. An electronic device is mounted within the cavity adjacent to the first surface of the package body. A lid extends over the opening and is attached to the package body. The lid may be transparent to light or opaque. A plurality of leads extend from the package body. Each such lead has a first portion adjacent the first surface of the package body, a second portion extending vertically adjacent to a peripheral side surface of the package body, and a third portion adjacent to and extending over the lid. The first portion of the lead is electrically connected to the electronic device by a bond wire or equivalent conductor. Such a package is particularly useful where the electronic device is selected from the group of an optical integrated circuit, a sensor integrated circuit, a pressure or gas sensitive integrated circuit, a gallium arsenide integrated circuit, a flip chip integrated circuit, and a micro machine, since the electronic device resides in a hollow cavity.
The package body may be formed of a molded encapsulant, such as injection molded plastic. In such a package, the first portion of each lead is embedded at the first surface of the package body. A lower surface of the embedded portion of the lead is exposed for electrical connection to a printed circuit board or another package. An upper surface of the embedded portion of the lead is exposed within the cavity. A bond wire or equivalent is electrically connected between the upper surface of the lead and the electronic device.
A plurality of such packages may be stacked one on top of the other so that the leads of a lower package will abut the leads of an upper package thereon. The respective bodies of the packages may include one or more keys that engage respective keyholes of the other package stacked therewith. The keys and keyholes ensure that the packages are precisely stacked, thereby assuring a correct and reliable electrical connection between the juxtaposed portions of the leads of the packages. Alternatively, one such package may be placed next to another on a printed circuit board so that the vertically extending portions of the leads of adjacent packages are juxtaposed for electrical interconnection.
These and other embodiments of the present invention, along with many of its many advantages and features, are presented in greater detail below and in the accompanying figures.